Various processes have been proposed to produce aromatic aldehydes such as p-tolualdehyde. Generally these methods involve reacting an alkyl-substituted aromatic hydrocarbon, such as toluene with carbon monoxide in the presence of some type of catalyst system.
G.B. Pat. No. 1,422,308 to Mitsubishi discloses a process for continuous production of aromatic aldehydes which comprises passing a reaction liquid comprising an aromatic hydrocarbon, hydrogen fluoride and boron trifluoride through a tubular reaction zone.
U.S. Pat. No. 2,485,237 discloses a process for the low temperature synthesis of aromatic aldehydes which also uses hydrogen fluoride and boron trifluoride, and which claims that this invention results in high yields of desired aldehyde products and, further, allows for easier separation of said product from catalyst.
In U.S. Pat. No. 3,948,998 another two-step process is disclosed for producing p-tolualdehyde which involves a catalyst system similar to that of the previously discussed references, but is characterized by reacting a preformed toluene-hydrogen floride-boron trifloride complex with carbon monoxide, thereby converting a portion of the toluene to p-tolualdehyde, and then adding the remainder of the BF.sub.3 to the reaction product and reacting the remainder of the toluene with carbon monoxide.
U.S. Pat. No. 4,218,403 discloses a catalyst system for producing aromatic aldehydes by reacting an alkylbenzene with carbon monoxide in the presence of a tantalum, niobium or antimony pentafluoride-hydrogen flouride catalyst system.
Gale, Gilbert and Osteryoung discuss the use of aluminum halide-alkylpyridinium halide mixtures as aprotic molten salt media in Inorg. Chem. 17, 2728 (1978).
In Inorganic Chemistry, Vol. 18, p. 1603, 1979, Gale and Osteryoung disclose the results of potentiometric work on the solvent acid-base properties of AlCl.sub.3 :N-butyl pyridinium chloride and an equilibrium constant for the dissociation reaction of AlCl.sub.4.sup.- was determined.
Gray and Maciel completed an Al NMR spectra study on liquid samples consisting of mixtures of AlCl.sub.3 and N-butylpyridinium chloride at various mole ratios and at various temperatures and have determined parameters for the AlCl.sub.4.sup.- and Al.sub.2 Cl.sub.7.sup.- ions in these melts and have estimated a chemical exchange rate. See J. Am. Chem. Soc., Vol. 103, p. 7147, 1981.
In the reaction of aromatic hydrocarbons with carbon monoxide in the presence of aluminum chloride and hydrogen chloride, stoichiometric amounts of the catalyst have generally been used. High yields of benzaldehyde and tolualdehyde can be obtained, but the scope of the reaction is limited since higher alkylbenzenes generally undergo scrambling via alkylation-dialkylation steps. Although carbonylation of aromatics also proceeds smoothly in the presence of HF/BF.sub.3, an obstacle to commercialization has been the difficulty of recycling the catalyst. See Chemicals from Syngas by R. A. Sheldon, D. Reidel Co., 1983, P. 122.
These methods suffer from one or more process deficiencies. For example, most of these processes resort to subambient temperatures, which of course involve some considerable process control. In other cases, large excesses of catalyst must necessarily be employed to carry out the synthesis to obtain appreciable yields. In still other instances, useful catalysts are highly corrosive leading to obvious problems. Lastly, in some the reaction is effected only at high pressures.